1. Field of the invention
The present invention relates to a method of cutting strips for a wound core of a transformer to which a cylindrical coil bobbin is applied.
2. Description of the Related Art
As the iron cores of transformers, wound cores in which a strip having excellent magnetic characteristics is wound in a ring shape are now used. For example, a wound core is obtained by winding a strip material on a winding spool to obtain a square, rectangular, stepwise, or circular cross-section (see: Kokoku Nos. 60-28375 and 61-22851, and Kokai No. 55-132027). To form this wound core, two split cylindrical coil bobbins are pressure welded at pressure welding faces thereof, and windings are wound on the coil bobbins.
In the prior art, a material having straight line sides is cut along one or more predetermined curves to obtain one or more series of strips, as explained later in detail. In this case, however, unused portions remain between the series of strips, which is disadvantageous from the viewpoint of an efficient material utilization.
In addition, the above-mentioned material is cut from a very wide silicon steel plate by using a plate slitter apparatus. In this case, however, since the plate-slitter apparatus is composed of a pair of shafts to which a large number of round blades are attached, the inner stress within the material in the width direction due to the curting operation cannot be released, and as a result, a large distortion is generated in the cut portion of the plate. Also due to the configuration of the above-mentioned pair of shafts, it is impossible to carry out an ideal shear process, thereby causing a scrathing of the cut material, and as a result, another large distortion is generated on the cut portions of the plate. Such a large distortion cannot be removed even by heat annealing after the curring operation, and thus the magnetic characteristics are impaired.
Further, for economic reasons, usually the center portion of a directional silicon steel plate, which has relatively excellent magnetic characteristics, is used mainly for heavy electrical components and the like, and the side portions of the directional silicon steel plate are used for strips for wound cores. However, the surfaces of the side portions of the plate are uneven due to the manufacturing process thereof and in addition, defects such as cracks and holes are often generated therein. Such defects should be naturally removed when cutting the material for a wound cores from the plate, but may remain due to the high manufacturing yield. These defects reduce the magnetic characteristics and other physical characteristics.
Still further, a material coil for wound cores is heavy, and the thickness thereof is very thin, i.e., 0.02 to 0.03 cm, and the surface thereof is slightly uneven. Therefore, even when a protection process such as a paper wrapping process is carried out, the end faces of the material coil in the width direction are easily damaged during shipping or transfer and the like. Such damage in the width direction leads to a malfunction of the magnetic characteristics and other physical characteristics.
The above-mentioned defective portions are generated on one side or on both sides of a cut material, thus creating a large number of defective products. Particularly, when it is impossible to determine that a product is defective from the appearance thereof, so that such a product is not determined to be defective until a final stage test, the manufacturing cost is increased.
Note that, to completely remove the above-mentioned defective portions from a material for strips, it is possible to reduce the width of the strip material, but this increases the manufacturing cost of the material.